Heretofore, feedback amplifiers are used as amplifiers for amplifying an input signal on feedback principles. The feedback amplifier is a circuit for amplifying an input signal while returning a portion of the input signal to the input terminal to superpose the returned signal on the original input signal, thus improving electric characteristics of the amplifier. A conventional current-feedback amplifier (preamplifier) for use in optical communication systems which have been developed rapidly with the advancement of the information communication technology in recent years will be described in detail below.
In a receiver used in an optical communication system, an optical signal received through an optical fiber is converted into a weak current signal by a photodiode or the like which serves as a photoelectric transducer. The current signal is converted into a voltage signal, which is amplified by a preamplifier. The preamplifier is required to have such characteristics as high sensitivity, a high gain, a wide band, and a wide dynamic range. To meet such a requirement, it has conventionally been the most general practice to use a current-feedback amplifier referred to as a transimpedance preamplifier.
FIG. 1 of the accompanying drawings shows, as the conventional feedback amplifier, a transimpedance amplifier using a field-effect transistor (FET). As shown in FIG. 1, input current 405 from a photoelectric transducer is input to an input terminal, and impedance-converted by a preamplifier into a voltage signal which is amplified through amplified signal path 403. Input current 405 that is input to the transimpedance amplifier and the output voltage from amplifying circuitry 410 of the transimpedance amplifier are related to each other in proportion to the product of the resistance of feedback resistor 401 and input current 405. According to the conventional arrangement, the resistance of feedback resistor 401 of the preamplifier is of a large value to provide a high gain for making the preamplifier highly sensitive to the weak input signal.
However, since there is a trade-off existing between the gain and the band of the conventional circuit, if the resistance of the feedback resistor is increased to achieve a high sensitivity, i.e., a high gain, then the band tends to be reduced. Another problem of the increased resistance of the feedback resistor is that a voltage amplitude due to an input current flowing through the feedback resistor increases, saturating the output amplitude, which is liable to distort the output waveform and cause an operation failure.